Zika virus was discovered in the Zika Forest of Uganda in 1947 and, despite its current prominence in the media, until this past year it was thought to be relatively benign. However, matters changed dramatically in 2015 when Zika virus emerged in Brazil, where it has since been associated with a striking surge in the incidence of infants born with microcephaly (abnormally small heads and brains). Astonishingly, the number of Brazilian children born with microcephaly allegedly jumped from 147 in 2014 to 4,783 cases as of February 2, 2016.

A December 2015 photo of a Brazilian infant with microcephaly.

The link between Brazil’s cases of microcephaly and in-utero infection with Zika virus was first implied by the geographic and temporal correspondence between the emergence of the virus in Brazil and the remarkable rise in the number of Brazilian cases of microcephaly (1). Subsequently, Brazilian health officials investigated 1,113 of their microcephaly cases and confirmed that 404 of them could be linked to Zika infection. In addition, actual Zika virus was detected in the amniotic fluid of several microcephalic fetuses, and anti-Zika virus antibodies were detected in the amniotic fluid of others—evidence that the virus indeed might cross the placenta and possibly infect the fetus (2). With those sorts of findings at hand, why do public health authorities still refer to the Zika/microcephaly link as merely suggested?

Some health officials and researchers claim that the surge in the Brazilian incidence of microcephaly, which allegedly occurred after the Zika outbreak, was merely an artifact, accounted for by a significant under-reporting of cases before the Zika outbreak. To that point, 21 Brazilian medical centers recently collaborated to reassess the head circumferences of 16,208 Brazilian neonates from Northeast Brazil (which contained the epicenter of the 2015 Zika epidemic) from late 2012 until the entry of the Zika virus into Brazil in mid-2014. As reported earlier this month (3), they found an astonishing incidence of microcephaly during that pre-Zika period; ranging from 2% to 8%. Moreover, and importantly, the number of affected babies actually peaked in 2014, before Zika virus had even been seen in Brazil! See Aside 1.

[Aside 1: These researchers also found an increased incidence of the most extreme cases of microcephaly in the last quarter of 2015, after Zika virus emerged in Brazil; a finding consistent with the possibility that something new was occurring after the Zika outbreak. See Note below.]

What might explain the under-reporting of microcephaly cases in Brazil, before its Zika outbreak? The authors of the collaborative study claim that it was mainly because there are no standardized criteria for diagnosing microcephaly—loosely defined as a condition, not an illness, characterized by an occipital-frontal head circumference smaller than expected for gestational age and gender. The absence of standardized diagnostic criteria was said to account for the discrepancies between the incidence of microcephaly found by the authors of the collaborative study and that recorded earlier in official sites. [A lack of consensus over the defining limits of microcephaly also accounts for the wide range in incidence of cases, of from 2% to 8%, in the collaborative study’s report.] See Note 1, below.

Interestingly, when these investigators narrowed the criteria for microcephaly to include only the most extreme cases (i.e., those neonates who fell into the lower third of all three criteria enumerated in Note 1; see below) the rates (0.04 percent to 1.9 percent) were still high, although now within the ranges reported elsewhere in the world. Consequently, the authors conclude: “It is possible that a high incidence of milder forms microcephaly has been occurring well before the current outbreak, but that only those extreme cases, with classical phenotypes, were being notified. And as the number of extreme cases increased over these past three or four months so did the awareness of health professionals who started to notify milder forms (3).”

The authors call attention to the fact that the clinical significance of the milder forms of microcephaly, which comprise the vast majority of reported cases, remains to be determined. They then assert that, “These observations highlight the need to review the situation carefully. Many questions need to be answered prior to concluding what problem we are facing, how it came about and which consequences it is likely to bring to the Brazilian population in years to come… We can only conclude that we are facing a new and challenging public health problem and that limited epidemiological and clinical data hinders conclusions at this early stage.”

In February 2016 another team of Brazilian researchers claimed that the lack of standardized diagnostic criteria for microcephaly led to an overestimate of the incidence of microcephaly after the Zika outbreak, rather than to an underestimatebefore the outbreak (5). That is, an increased incidence of microcephaly was reported after the outbreak because normal children, whose heads were small, actually comprised the majority of alleged cases.

Here is another reason for questioning the link between Zinka infection and microcephaly. Whereas Zika virus was discovered in Uganda more than 60 years ago, and has since spread to more than a dozen countries, the recent surge of microcephaly in Brazil is the only case-in-point, anywhere in the world, in which microcephaly has been associated with Zika virus. Brazil’s neighbor, Colombia, is the world’s second-most Zika-affected country, with around 20,000 confirmed Zika infections. Yet while more than 2,000 of the Columbian Zika infections were of pregnant women, none of their fetuses were diagnosed with microcephaly.

Moreover, and in contrast to the lack of an association between Zika and microcephaly outside of Brazil, the Zika outbreak has been associated with a surge in Guillain-Barré syndrome (a temporary paralysis) in Colombia, El Salvador, Suriname, Venezuela, and French Polynesia, as well as in Brazil. The apparent universal association of Zika with Guillain-Barré syndrome, but not with microcephaly, might be taken as an argument against an etiologic role for Zika virus in microcephaly.

Yet, even in the case of Guillain-Barré syndrome, the World Health Organization considers the link to Zika to be tenuous; in part because other arthropod-borne viruses, including dengue, chikungunya and Zika viruses, have all been circulating simultaneously in the Americas (5). [Guillain-Barré syndrome occurs after infection by a variety of pathogens, including dengue and chikungunya viruses, which are related to Zika.] Likewise, the circulation of these other arthropod-borne viruses in areas hard hit by Zika raises the possibility that they might be involved in microcephaly.

So, where do we stand? Uncertainty regarding the connection between Zika and microcephaly underscores the need for clinicians to come to a consensus regarding the criteria that define that condition. Moreover, since most of the mothers who participated in earlier epidemiologic studies were not tested for Zika (even if they might have been infected), and since Zika causes a relatively mild illness that often goes undetected, and since other pathogenic arthropod-borne viruses also circulate in areas in which Zika is prevalent, there is a crucial need for a convenient and unambiguous molecular diagnostic test to identify these infections. See Aside 2.

[Aside 2: A convenient diagnostic test for Zika virus is also needed to protect the blood supply in countries were the virus is spreading by local transmission. Many of these countries are poverty stricken and already suffer from low donation rates and dwindling blood supplies. They can not long depend on outside sources of blood for their transfusions.]

But even with standardized diagnostic criteria for microcephaly, as well as accurate tests for infection, important gaps still remain in our knowledge of Zika virus—gaps that must be filled before the role of the virus in microcephaly can be known with certainty.

But isn’t the presence of Zika virus in the amniotic fluid of microcephalic fetuses proof enough? It isn’t because the handful of other viruses that are able to cross the human placenta (e.g. rubella, cytomegalovirus) are not known to cause microcephaly at the extraordinary rates currently being reported in Brazil. Thus, it is necessary to establish with certainty that Zika virus does, in fact, target and harm the fetal brain. [If it were ascertained that Zika virus does target the fetal brain, then it will be important to know how much time elapses after the mother is infected, before the virus can strike the fetus. Moreover, it will be important to determine whether the fetal brain is at risk to Zika during all, or during only some stages of its development.] See Aside 3.

[Aside 3: The MMR vaccine largely protects American children against congenital rubella. However, worldwide, more than 100,000 children continue to be born each year with this condition. Cytomegalovirus, for which there is no vaccine, causes at least 5,000 cases of birth defects each year in the United States alone.]

Importantly, despite the reservations noted above, many, if not most researchers believe that Zika virus indeed is the agent behind a very real surge in the incidence of microcephaly in Brazil. Moreover, the World Health Organization declared that the rise in microcephaly constitutes a global health emergency. Thus, while we await more rigorous proof of the Zika/microcephaly connection, it remains essential to act as though it were real.

Note 1:

“In this study (3), classification of microcephaly was based on three different criteria, as follows:

Neonates were classified with microcephaly according to each one of the three criteria.
A separate group was created for those who fulfilled all three criteria. Finally, those who fell into the lower third in each criterion were grouped as extreme cases of microcephaly.”

Ebola, MERS, and Hepatitis C viruses dominated virology news during the past year (2015). Now, early in 2016, Zika virus has taken center stage. The reasons are clear. This once seemingly innocuous virus, initially restricted to Equatorial Africa, has of late spread to the Western Hemisphere, and is now suspected (but not proven) to cause microcephaly—an otherwise rare condition in which babies have unusually small heads and incomplete brain development—in transplacentally infected fetuses of infected pregnant woman. Moreover there is evidence which links Zika virus to Guillain–Barré syndrome—a potentially severe autoimmune attack on peripheral nerves that may occur after signs of a viral infection. We begin with some background.

Zika virus is a member of the flavivirus family of plus-strand RNA viruses. The family also includes several notable human pathogens, including yellow fever, dengue, hepatitis C, and West Nile viruses. Like most other flaviviruses, Zika virus too is spread by an arthropod vector; in this instance Aedes mosquitoes. 80% of Zika virus infections are asymptomatic and, prior to recent developments, symptomatic infections were seen as mild, acute febrile illnesses, similar to dengue.

Zika virus was discovered by accident in the Zika Forest of Uganda in 1947. The discovery was made by scientists who had been studying yellow fever. They isolated Zika virus from one of their rhesus macaques, which was suffering from an unknown fever. The following year the same virus was found in Aedes mosquitos from the same Ugandan forest, thus identifying the mosquito as a vector for Zika virus. Zika virus was detected for the first time in humans in 1954, in Nigeria.

The Aedes aegypti mosquito, the Zika virus vector

Until recently, Zika virus infections were rare and were reported only within equatorial Africa and Southeast Asia. Then, in 2007, an outbreak occurred in Yap Island, Micronesia. The Yap Island Zika outbreak was the first one outside of Africa and Asia. None of the Yap Island cases, which included 49 in which Zinka virus was confirmed by the presence of Zinka RNA, resulted in either hospitalization or death.

The Yap Island outbreak was followed by epidemics in Polynesia, Easter Island, the Cook Islands and New Caledonia. The Polynesian outbreak was notable for being the first in which Zika infection was associated with Guillain–Barré syndrome.

Concern over Zika virus was heightened, particularly in the Americas, when, in April 2015, a large and still ongoing outbreak of Zika virus occurred in Brazil. The Brazilian outbreak marked the first appearance of Zika virus in the Western Hemisphere. It is not clear how Zika virus made its way to Brazil, but it is widely believed that the virus made the leap from Polynesia to Brazil during the 2014 World Cup soccer tournament.

Apprehension over Zika virus increased in November 2015 when the virus was isolated from a Brazilian newborn with microcephaly. By December 2015 many more cases of this generally rare disorder were reported. The European Center for Disease Prevention and Control then warned of a possible association between Zika virus infection and congenital microcephaly, and with Guillain–Barré syndrome as well.

More than a million Brazilian people since been infected with Zika virus, and the number of Brazilian children born with microcephaly jumped from 147 in 2014 to nearly 4,000 in 2015. There is no anti-Zika vaccine, nor is there an effective therapy.

The first Zika virus-associated case of microcephaly in the United States occurred in early January 2016 in a baby born in Oahu, Hawaii. The baby and its mother each tested positive for a past Zinka infection; probably acquired in May 2015 when the mother, then pregnant, had been traveling in Brazil.

On January 24, 2016 the World Health Organization warned that Zika virus will likely spread to every nation in the Western Hemisphere (possibly excepting Canada and Chile), since its Aedes aegypti vector can thrive in tropical and sub-tropical climates here. The Aedes mosquito has long been present in the United States, ranging as far north as New York and west into Indiana and Illinois. [An earlier posting reported that Aedes aegypti may have been brought from Africa to the New World by slave ships in 1596 (1). Mosquito larvae, present in the water casks of the sailing ships of the day, also carried yellow fever to the New World.]

Global concern over the Brazilian Zika outbreak was heightened by the fact that Brazil is scheduled to host the Olympic Games this summer, and about 500,000 people are expected to attend from all over the world, including 200,000 Americans. Some of these attendees will, of course, be bringing the virus back to their home countries.

Brazilian officials no doubt are concerned that their Zika outbreak will affect attendance at the upcoming Olympic Games. Consequently, commercial considerations may be one of the motives behind Brazil’s extensive campaign to eradicate its mosquitoes. Unfortunately, standard approaches, such as using insecticides and removing standing water where mosquitoes breed, have not done the job. Thus, the Brazilian Zika outbreak may not be under control by the start of the Olympic Games. [Brazil also experienced more than 1.6 million cases of dengue during 2015, with 863 people dying from the disease, underscoring that the Aedes mosquito vector is not well contained in that country.]

The failure of Brazilian vector-control approaches suggests that new strategies may be needed to contain the outbreak. Apropos that, this past January Colombia began releasing mosquitoes treated with bacteria, which are hoped might limit the mosquitoes’ capacity to spread disease. Note that insecticides have limited effectiveness. Not only are they toxic to humans, but after decades of overexposure to them, many mosquitoes are now resistant.

Zika virus is now present in the continental United States. Thus, it is timely to consider how grave a threat Zika virus might impose here. To that point, consider that yellow fever, dengue and chikungunya viruses are dangerous pathogens that also are spread by Aedes mosquitoes. Yet these viruses are not regarded as important threats in the United States. That is so because our vector control measures have thus far been able to contain them. Those measures might likewise be expected to contain local transmission of Zika virus here.

But, what if Zika virus has a mode of transmission other than via its mosquito vector? To that point, there is a single reported case of Zika transmission via a blood transfusion. Also, it was suggested that Zika virus might have a sexual route of transmission, as per the finding of high levels of the virus in the semen of a man from French Polynesia. In addition, there is a report of an American scientist, Brian D. Foy, who contracted Zika virus while working in Senegal in 2008, and who transmitted the virus to his wife after returning home (2). Serologic analyses of the couple’s convalescent serum confirmed that they had been infected with Zika. Sexual transmission is implicated in this instance since neither Foy nor his wife passed the infection to their children or to other close relatives. Moreover, Foy and his wife observed signs of hematospermia (red–brown fluid in his ejaculate).

Foy notes in his scientific report (2), “If sexual transmission could be verified in subsequent studies, this would have major implications toward the epidemiology of Zika virus and possibly other arthropod-borne flaviviruses.” [Human sexual transmission of an arthropod-borne virus has not yet been documented.] Foy has been trying to get funds to investigate sexual transmission of Zika. However, according to a January 26, 2016 article in the N.Y. Times, the CDC says that the “theoretical risk” of sexual transmission in the above instances is insufficient to justify a warning (and funding?). But, see the following paragraph.

As I’m sitting at my computer on the evening of February 2, 2016, NPR, CNN, BBC News, the N.Y. Times, etc., are reporting a case of Zika virus infection in Texas that appears to have been sexually transmitted. According to the Dallas County Health and Human Services Department, a patient with the Zika virus was infected after having sex with someone who returned from Venezuela, where Zika is circulating. The CDC appears to give credence to the Texas report, since it quickly responded to it by advising men having sex after traveling to these areas to “consider” wearing condoms, and advised pregnant women to avoid “contact with semen” from men recently exposed to the virus.

Sexual transmission will probably account for only a very small fraction of Zika cases, but that isn’t known for certain. As in instances of mosquito-borne transmission, its contribution will depend in part on how long the virus might persist in infected individuals.

Since the vast majority of Zika virus infections are likely transmitted via its mosquito vector, and since Zika virus mainly threatens fetuses infected in utero, the most severe consequences of Zika virus infection can be largely avoided if pregnant women, or women planning to become pregnant, avoid traveling to places where Zika virus remains prevalent (a fact which doesn’t help individuals living in those regions). For that reason, on January 15, 2016, the United States Centers for Disease Control and Prevention (CDC) released a list of countries—Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela, and Puerto Rico—where mosquitoes are spreading the Zika virus, and which pregnant women should avoid at this time. On February 1, 2016, the World Health Organization added Costa Rica and Jamaica.

Political and commercial considerations may have been behind the Brazilian minister of tourism taking exception to the CDC’s warning, claiming that measures adapted by Brazilian health authorities are bringing the Zika outbreak under control, and that Brazil is, in fact, a safe destination for pregnant women. The Brazilian health minister added, “Zika virus doesn’t worry us…,” calling it a “benign disease.” Those pronouncements were made despite the fact that Brazilian health authorities were at the same time investigating more than 3,500 cases of microcephaly. But at least some Brazilian health professionals did endorse the CDC announcement.

On February 1, 2016 the World Health Organization took the further step of declaring that Zika virus and its suspected link to birth defects constitute an international public health emergency. Yet the WHO stopped short of advising pregnant women not to travel to affected regions. Some public health experts claimed that the WHO’s silence on that point was more about politics than public health. Any travel ban—even one aimed only at pregnant women—would be embarrassing and costly to Brazil, which is moving ahead with its plans to host the Olympic Games this summer. And, while there have been calls to cancel, postpone, or move the Rio games, the International Olympic Committee (IOC) hasn’t expressed any concerns over the Games taking place as planned.

Meanwhile, the governments of Columbia, El Salvador, Ecuador, and Jamaica have taken the rather extraordinary step of recommending that women avoid getting pregnant until the Zika outbreak might be brought under control in their countries. This advisory was not well received by many El Salvadoran women, especially in view of the strict abortion laws and high levels of sexual violence against women in that country.

And, as I’m putting the final touches on this piece, an article in today’s (February 4, 2016) N.Y. Times reports that the Zika virus/microcephaly link is causing a fierce debate in Brazil over its strict abortion laws; under which abortion is illegal under most circumstances. [Remarkably, Brazil’s strict abortion laws are actually less restrictive than those in other Latin American countries.] Some Brazilian doctors are already seeing pregnant women who are seeking abortions because they fear microcephaly. Yet conservative Brazilian lawmakers actually want to make the restrictions against abortion more stringent than they already are. [The Times article says that their position reflects “the influence of Roman Catholic leaders and the increasingly powerful preachers at the helm of a growing evangelical Christian movement.”] Regardless, individuals on both sides of the debate might be troubled by the fact that microcephaly can not be detected by ultrasound scans until the end of the second trimester, when the “child” is already very much formed. Moreover, the criteria for diagnosing microcephaly are rather non-specific, and it is difficult to predict what its consequences might be.

A crucially important question regarding Zika virus concerns determining its true pathologic potential, particularly its role in microcephaly—a role that is strongly inferred (but not proven) by the geographic and temporal relationship between microcephaly and Zika infection. To that point, no increase in microcephaly has been linked to Zika virus outside of Brazil. For instance, Colombia is the second-most Zika-affected country, with around 20,000 confirmed cases. More than 2,000 of the Columbian cases have been pregnant women. Yet none of their fetuses have been diagnosed with microcephaly.

Did Zika virus become an etiologic agent for microcephaly only after reaching Brazil? If so, how did that happen? Was it because of the emergence of a new strain of the virus? Or, does Zika virus cause microcephaly only if the mother has had a previous infection, like dengue? Alternatively, was the link simply missed in the past because, until now, the virus has not invaded a country where there are a large enough number of non-immune individuals, who also are living under conditions that are ideal for the virus to spread? Or, were previous cases merely under-reported, such that the 147 Brazilian cases in 2014 were a vast underestimate?

The flip side is that the current extraordinarily high number of reported cases of microcephaly in Brazil might merely be due to a heightened awareness of that condition; a possibility that is favored by some Brazilian officials. A supporting argument is that the criteria for diagnosing microcephaly are relatively unspecific. However, others point out that physicians were reporting a rise in cases as early as November 2015, before the increased attention from health authorities and the media.

Another unexplained yet key factor is the unusually severe congenital deformities—extensive loss of brain tissue, unusually smooth, wrinkleless brains, many calcium deposits, and smaller cerebellums—seen in the Brazilian microcephaly cases. These features are not characteristic of microcephaly caused by other pathogens, such as toxoplasmosis, cytomegalovirus, or rubella.

And, presuming that Zika virus indeed causes microcephaly, how or why is it able to cross the human placenta and enter the fetal brain? [In December 2015, the Pan American Health Organization reported that Zika virus RNA was identified by reverse transcription-polymerase chain reaction (RT-PCR) in amniotic fluid samples from two pregnant women whose fetuses were found to have microcephaly by prenatal ultrasound. Moreover, Zika virus RNA was identified in multiple fetal body tissues, including the brain of an infant with microcephaly (3).] Remarkably, only a handful of viruses cross the human placenta and infect the fetus with any notable frequency (4). These include rubella virus, cytomegalovirus, and HIV; none of which is related to Zika virus. Yellow fever, dengue, and West Nile viruses, which are related to Zika virus, are not known to harm embryos.

Since most Zika virus infections are either asymptomatic, or present with flu-like symptoms that mimic other infections, a rapid diagnostic test for Zika infection is needed to accurately measure the prevalence of the virus in a population, and to measure its spread. Such a test might also help sort out whether the Brazilian microcephaly cases indeed have been due to Zika, rather than to another virus, such as the related dengue virus. Efforts are currently underway to develop Zika-specific immunological reagents for these purposes.

Vaccine researchers say that a vaccine against Zika virus may be available for testing by the end of 2016. But, even if the vaccine were effective, how long might it take for it to gain approval?

Meanwhile, an increasing, but still small number of Zika virus infections are being detected in the continental United States. With the exception of the Texas case noted above, all cases have thus far involved travelers who recently returned from overseas. Thus, with the exception of the Texas case, there is no evidence yet for local transmission here. But that well might change as summer approaches.

So, Zika now joins Lyme, West Nile, Chagas, dengue, and chikungunya on the list of recently emergent arthropod-borne diseases. Still, as we’ve noted, it is not yet clear how much of a threat Zika virus actually poses. Regardless, until that is known, it will be necessary to prepare for the worst. Even if the threat of Zika has been vastly overblown, progress towards its containment will pay important dividends in the containment of established threats, such as dengue and chikungunya.

And, if Zika is indeed a dangerous pathogen that is responsible for severe birth defects, then current conditions—global warming, more people traveling worldwide on jet airliners, cities in tropical countries becoming larger and ever more crowded—don’t portend well for the future. Stand by for new developments.

References:

1. The Struggle Against Yellow Fever: Featuring Walter Reed and Max Theiler, Posted on the blog May 13, 2014.

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I am now a retired professor emeritus of Microbiology at the University of Massachusetts. Teaching virology has been a most rewarding aspect of my career. I especially enjoyed enlivening my lectures with a variety of relevant anecdotes.

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Based on my experiences teaching virology for more than 35 years, I wrote Virology: Molecular Biology and Pathogenesis (ASM Press; 2010). For info on adopting or buying this textbook, please visit the publisher site: http://www.asmscience.org/content/book/10.1128/9781555814533